The present invention relates to an electric parking brake system.
An electric parking brake system including an electric parking brake and a controller is conventionally known. The electric parking brake is driven by a motor to apply braking force to wheels.
As one example, Japanese National Phase Laid-Open Patent Publication No. 2001-523616 discloses an electric parking brake system including a brake actuator, an electronic control unit that includes a plurality of controllers for separately processing input signals to convert the signals into the corresponding output signals, and a plurality of electric parking brakes that are actuated in accordance with the output signals. The electric parking brake system further comprises a voter that is connected downstream of the controllers and to give decisions based on the majority vote.
According to the above structure, even when an electric failure occurs in a circuit including one of the controllers, at least one electric parking brake can be actuated by a circuit including another controller. This ensures execution of an emergency operation mode. Further, even when, for example, the output signals from the controllers differ from one another, the voter determines a final output signal with the majority vote. This enables each electric parking brake to be provided with a stable output signal.
As another example, Japanese National Phase Laid-Open Patent Publication No. 2002-529314 discloses an electric parking brake system including an input unit for receiving a request from a driver, and an electronic control unit for detecting the driver's request based on a signal output from the input unit and controlling a parking brake. The input unit outputs a plurality of redundant signals to the electronic control unit. The electric parking brake system further includes an evaluation unit for detecting an electric failure occurring in the input unit. A resistor is connected to each of signal lines, supply lines, and ground lines provided in the input unit. The evaluation unit detects an electric failure in the input unit by measuring the potential at each resistor.
According to the structure disclosed in Japanese National Stage Laid-Open Patent Publication No. 2002-529314, the evaluation unit detects such failures occurring in the input unit as a break in a wiring, a switch contact failure caused by corrosion, and a short circuit in a supply line, based on the measured potential at each resistor and the redundant signals. This structure enables failures to be readily coped with.
According to the structure disclosed in Japanese Laid-Open National Phase Patent Publication No. 2001-523616, when an electric failure occurs in a circuit including one of the controllers, the electric parking brake connected to the failed circuit cannot be actuated. In an emergency operation mode, specifically, in a parking-brake applying operation where one of the electric parking brakes is not functioning, reliability and stability of the braking performance is degraded. Further, at least three or more controllers in an odd number are required to enable the voter to function. This complicates the structure and increases the cost of the electric parking brake system.
In the electric parking brake system disclosed in Japanese National Phase Laid-Open Patent Publication No. 2002-529314, the electric parking brake is arranged at such a location to which vibrations are easily transmitted when the vehicle is traveling. Further, the electric parking brake arranged at the location may become wet. Accordingly, a failure is more likely to occur in the electronic control unit and the electric parking brake than in the input unit.
Despite this, the electronic control unit, which can detect an electric failure in the input unit, cannot detect a failure in its control target electric parking brake. The electric parking brake system fails to detect a failure occurring both in the electric parking brake and on a path to supply power to the electric parking brake at an early stage. Thus, such a failure cannot be coped with immediately.